pubs.acs.org/Langmuir © 2009 American Chemical Society
Multilayer Nanostructured Porphyrin Arrays Constructed by Layer-by-Layer Self-Assembly Arthur R. G. Smith,† Jeremy L. Ruggles,*,† Aimin Yu,‡ and Ian R. Gentle† †
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Australia 4072, and Division of Science, School of Chemical and Mathematical Sciences, Murdoch University, Perth, Western Australia
‡
Received March 21, 2009. Revised Manuscript Received May 27, 2009 UV-vis absorption, atomic force microscopy (AFM), contact angle, and X-ray reflectivity experiments were performed on thin films deposited on crystalline silicon substrates as alternating layers of a porphyrin with anionic functionality, tetra-5,10,15,20-(4-sulfonatophenyl)porphine (TSPP) or the metalated version, Cu(II)TSPP, and the cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (PDDA). The films were made by dipping in alternating aqueous solutions containing film components (layer-by-layer deposition). Modeling of the X-ray reflectivity data revealed differences in the films’ thickness depending on the method of film deposition. An unusual decrease in film thickness after each polyelectrolyte dip was also observed for films using TSSP. UV-vis measurements revealed that a similar amount of TSSP was included within films despite the method of formation. UV-vis measurements also revealed the presence of free-base, H-aggregate, and J-aggregate forms of the porphyrin after TSPP dipping, and the subsequent disappearance of the J-aggregate after dipping in the PDDA solution. A model of film formation was proposed on the basis of the concept of two different types of porphyrin aggregates being present after dipping in porphyrin solution. A layer of porphyrin molecules initially attach to the Si surface such that the planar molecules are arranged side by side as H-aggregates with an excess of J-aggregated material on top. The J-aggregate is then removed and replaced by a layer of PDDA. A change in contact angle of 14 was observed between porphyrin and polyelectrolyte layers due to the more hydrophobic nature of the polymer. The presence of the J-aggregate was confirmed in AFM images obtained from the porphyrin layer. Exposure of the films to solutions of alternating pHs of 10 and 1.8 resulted in reproducible switching of the UV-vis spectra, indicating a possible sensing application.
Introduction Porphyrins and their derivatives are found in many important biological processes, including photosynthesis and oxygen transport by hemoglobin.1-4 They have attracted significant attention for their possible applications that take advantage of their useful electronic and optical properties. Examples include their use in artificial photosynthesis, as solar cells,5-13 as gas sensors,14-16 in nonlinear optics,17-21 in molecular electronics,22,23 in smart *To whom correspondence should be addressed. Telephone: þ61 7 3346 7532. Fax: þ61 7 3365 4299. E-mail:
[email protected]. (1) Deisenhofer, J.; et al. Nature 1985, 318(6047), 618–624. (2) Momenteau, M.; Reed, C. A. Chem. Rev. 1994, 94(3), 659–698. (3) Akins, D. L.; Zhu, H. R.; Guo, C. J. Phys. Chem. 1994, 98(14), 3612–3618. (4) Falk, J. E. Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier Scientific Publishing Co.: Amsterdam, 1975. (5) Morisue, M.; et al. Chem.;Eur. J. 2005, 11(19), 5563–5574. (6) Kaschak, D. M.; et al. J. Am. Chem. Soc. 1999, 121(14), 3435–3445. (7) Tsuchiya, Y.; et al. J. Mater. Chem. 2004, 14(7), 1128–1131. (8) Martinson, A. B. F.; et al. J. Electrochem. Soc. 2006, 153(3), A527–A532. (9) Chukharev, V.; et al. Langmuir 2005, 21(14), 6385–6391. (10) O’Regan, B.; Gratzel, M. Nature 1991, 353(6346), 737–740. (11) Umeyama, T.; Imahori, H. Photosynth. Res. 2006, 87(1), 63–71. (12) El-Nahass, M. M.; et al. Thin Solid Films 2005, 492(1-2), 290–297. (13) Wasielewski, M. R. Chem. Rev. 1992, 92(3), 435–461. (14) Han, B. H.; Manners, I.; Winnik, M. A. Chem. Mater. 2005, 17(12), 3160– 3171. (15) Kurtikyan, T. S.; et al. Eur. J. Inorg. Chem. 2003, No.10, 1861–1865. (16) Ding, H.; et al. Thin Solid Films 2000, 379(1-2), 279–286. (17) Collini, E.; et al. J. Mater. Chem. 2006, 16(16), 1573–1578. (18) Jiang, L.; et al. J. Phys. Chem. B 2005, 109(13), 6311–6315. (19) Fox, J. M.; et al. J. Am. Chem. Soc. 1999, 121(14), 3453–3459. (20) Jiang, L.; et al. Thin Solid Films 2006, 496(2), 311–316. (21) Truong, K. D.; et al. Thin Solid Films 1994, 244(1-2), 981–984. (22) Kang, B. K.; et al. Mater. Sci. Eng., C 2006, 26(5-7), 1023–1027. (23) Chowdhury, A.; et al. Solid State Commun. 1998, 107(12), 725–729. (24) Kovaric, B. C.; et al. J. Am. Chem. Soc. 2006, 128(13), 4166–4167. (25) Ciszewski, A.; Milczarek, G. J. Electroanal. Chem. 1999, 469(1), 18–26. (26) Vangalen, D. A.; Majda, M. Anal. Chem. 1988, 60(15), 1549–1553.
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biomaterials,24 in catalysis,25,26 and even in photochemical hydrogen production.27 Controlling the natural tendency of porphyrins to aggregate and in so doing creating ordered layers of coplanar assembled porphyrins is advantageous for their practical use in light-harvesting devices. Applications which call for highly ordered nanostructured porphyrin arrays include nonlinear optical structures as devices for modifying incident light waves,18 two-dimensional diffraction applications, using metal-centered porphyrins to evenly distribute metals as a catalytic medium,26 obtaining an even distribution of metalloporphyrins on a surface to maximize catalytic efficiency and promote controlled electron transfer by having ordered networks of porphyrins,22 and as building blocks for molecular or protein recognition systems.28 In this work, we show how it is possible to create arrays of selected forms of the aggregate in an ordered arrangement as a result of the layer-by-layer (LBL) process. UV-vis spectroscopy of porphyrins reveals a strong “Soret” band visible around 400 nm due to the extreme conjugation of the macrocycle. This aromatic nature of the porphyrin unit gives it great thermal stability, contributes to its ability to obtain a 2þ charged state (through protonation of the central ring),4 and is also responsible for the tendency of porphyrins to form aggregates4,29-31 due to π-π interactions between the porphyrin central rings.4,30,32 The most common mode of aggregation is (27) Amao, Y.; Tomonou, Y.; Okura, I. Sol. Energy Mater. Sol. Cells 2003, 79(1), 103–111. (28) Zhou, H.; et al. J. Am. Chem. Soc. 2006, 128(7), 2421–2425. (29) Lauer, M. E.; Fuhrhop, J. H. Langmuir 2004, 20(19), 8321–8328. (30) Snitka, V.; Rackaitis, M.; Rodaite, R. Sens. Actuators, B 2005, 109(1), 159– 166. (31) Zhang, H.; et al. Colloids Surf., A 2005, 257-258, 291–294. (32) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112(14), 5525–5534.
Published on Web 07/02/2009
DOI: 10.1021/la900953a
9873
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Figure 2. Solution spectroscopy of 5 μM TSPP with 0.05 mg/mL PDDA.
Figure 1. Solution spectroscopy of 5 μM (a) TSPP and (b) CuTSPP.
This method has been utilized for the construction of alternating multilayers of charged porphyrin and polyelectrolytes in the work by Ariga et al.35 It was found that the adsorption stability of the layers, particularly the porphyrin layer, was greatly affected by the characteristics of the polyelectrolyte used. It was established that the strong polyelectrolyte poly(ethyleneimine) (PEI) caused the porphyrin layer to desorb during subsequent dipping for further layer formation and that poly (diallyldimethylammonium chloride) (PDDA) did not have the same effect. Further work on this layer-by-layer technique has been conducted,36 with particular interest paid to the nonlinear optical properties of the porphyrin/polyelectrolyte multilayer stack.17,18,20 The characterization of the surface and orientation of the porphyrins for much of this work relied on UV-vis absorption measurements (achieving a linear increase in absorption with each bilayer deposition)17,18,20 combined with some simple X-ray diffraction techniques for estimating the total thickness of the multilayer. In this work, the LBL method has been used to form multilayer assemblies of charged porphyrin and polyelectrolyte on quartz slides and Si Æ111æ wafers. Detailed structural information was obtained through a combination of X-ray reflectometry (XRR), used to probe the thickness and roughness of layering achieved, and UV-vis adsorption spectroscopy, used to monitor the aggregation states of the porphyrins. Rather than the simple linear growth that is normally seen in LBL deposition, these films were observed to change thickness in a “sawtooth” fashion, a fact that is explained by changes in the aggregation of the outermost layer while in contact with solution.
the so-called J-type,30 in which the porphyrins align in a slipped face-to-face orientation30 and can occur under acidic conditions, or in the presence of organic or inorganic cations.33 The other common type of aggregate observed in porphyrins is H-type, where porphyrins stack directly over each other. Aggregation can be observed with UV-vis spectroscopy, as the characteristic strong Soret band is shifted upon aggregation.3 Traditionally, thin films containing porphyrins have been manufactured by using Langmuir-Blodgett (LB) or self-assembled monolayer (SAM) techniques. Both methods have their inherent weaknesses; LB technology places constraints on the types of molecules that can be used, requiring molecules that are insoluble in water and can form monolayer films. SAM techniques require a strong interaction between the monolayer and substrate, such as that found between thiols and gold substrates. The LBL deposition technique, based on sequential layering of oppositely charged adsorbates to form multilayer films, can overcome some of these problems. LBL self-assembly of polyelectrolytes was first investigated by Decher et al.34 and the seminal work on the topic published in Science in 1997.34 Briefly, a charged substrate is placed into a solution of an oppositely charged polyelectrolyte. The polyelectrolyte adsorbs electrostatically to the surface, and the surface is rinsed and then placed in another solution containing a polyelectrolyte of opposite charge to the first, which then adsorbs to form a second layer. This process can be continuously repeated until the desired number of layers is formed. The technique has advantages of simple preparative procedures, the availability of many water-soluble compounds, control over layer thickness, and the fact that a wide variety of substrates can be used.
Materials. The polyelectrolytes used in this investigation, poly (ethyleneimine) (PEI, 50 wt % solution in H2O, Mw=750000), poly(4-styrene-sulfonate) (PSS, Mw ∼ 70000), and poly(diallyldimethylammonium chloride) (PDDA, 20 wt % solution in H2O, Mw = 100000-200000), were obtained from Aldrich and used as received. The porphyrins, tetra(4-sulfonatophenyl)porphine (TSPP) and Cu(II)TSPP, were obtained from Frontier Scientific and used as received. Low-background quartz slides (12 mm 35 mm) were obtained from H. A. Groiss & Co. (Melbourne, Australia). Silicon wafers Æ111æ were obtained from Si-Mat Silicon Materials, and these were used as appropriate for UV-vis, X-ray reflectometry, atomic force microscopy (AFM), and contact angle measurements.
(33) Fujii, Y.; Tsukahara, Y.; Wada, Y. Bull. Chem. Soc. Jpn. 2006, 79(4), 561– 568. (34) Decher, G. Science 1997, 277(5330), 1232–1237.
(35) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119(9), 2224– 2231. (36) Van Patten, P. G.; Shreve, A. P.; Donohoe, R. J. J. Phys. Chem. B 2000, 104(25), 5986–5992.
9874 DOI: 10.1021/la900953a
Experimental Section
Langmuir 2009, 25(17), 9873–9878
Smith et al.
Article Table 1. Results of the X-ray Reflectivity Modeling and Contact Angle Measurementsa
film TSPP/PDDA CuTSPP/PDDA
layer material
thickness (A˚)
scattering length density (10-6 A˚-2)
contact angle (deg)
PEI/PSS/PDDA TSPP PDDA PEI/PSS/PDDA CuTSPP PDDA
21.8 ( 0.1 24.5 ( 1.5 -6.4 ( 0.8 23.5 ( 0.2 5.7 ( 0.4 2.9 ( 0.3
9.8 ( 0.1 11.6 ( 0.1 10.9 ( 0.2 8.7 ( 0.1 10.7 ( 0.1 10.7 ( 0.2
33 ( 1 47 ( 3 -
a All reflectivity profiles were modeled as one-layer films. The thickness of the porphyrin and PDDA layers is the average change with that step (standard error shown). The uncertainty in the modeled thickness and SLD was
